1. Field of the Invention
The present invention relates to an indirect heating furnace suitable for heat treatment of solid inorganic substances, more particularly, for high-temperature heat treatment in cases where direct contact of the solid inorganic substance with high-temperature combustion gas is unfavorable.
2. Description of the Related Arts
In a solid heating process in which direct contact of a substance to be heated with high-temperature combustion gas is unfavorable, an indirect heating furnace (also called an external heating furnace) is generally used.
However, conventional Indirect heating furnaces use metallic shells for reaction tubes. This limits the conventional indirect heating furnace to heating processes requiring temperatures no higher than 900xc2x0 C.
To overcome this problem, U.S. Pat. No. 5,846,072 (Patent Literature 1) has described that an indirect heating furnace in which the reaction tube is made of ceramics and a screw conveyor is provided in the reaction tube to convey a solid substance to be heated, by which the Indirect heating furnace can be applied to processes requiring temperatures above 900xc2x0 C.
FIG. 4 shows an example of a general system configuration in the case where a substance to be heated is heat-treated using such an indirect heating furnace provided with a ceramic reaction tube. In the example shown in FIG. 4, a case where lime is burned (thermal decomposition of limestone) is illustrated.
In FIG. 4, limestone charged into a supply chamber 120 through a raw material charge port 110 is transferred in a reaction tube 140 by the rotation of a screw conveyor 130, and conveyed into an outlet chamber 150. The limestone is subjected to heat treatment during the time when it is passed through the reaction tube 140 toward the outlet chamber 150. The heat-treated product in the outlet chamber 150 is discharged to the outside of the furnace through a chute 160.
Outside the tube, high-temperature combustion gas, which is generated by a combustion burner 170, is introduced into the furnace through a gas introduction port 180 to heat the limestone in the reaction tube 140 indirectly via the wall of the reaction tube 140, and is discharged from the furnace through an exhaust port 190. The furnace exhaust gas discharged through the exhaust port 190 is sent to an air preheater 200 to preheat combustion air supplied to the combustion burner 170, by which the heating value of the furnace exhaust gas is utilized effectively.
In the indirect heating furnace as shown in FIG. 4, heating is accomplished in three steps, in that heat is transferred from the high-temperature combustion gas supplied into the furnace to the outside wall of the reaction tube 140, it is conducted through the tube wall, and then the heat is transferred from the tube wall to limestone, which is a substance to be heated. Because of this, the heat transfer efficiency is poor, and the temperature of furnace exhaust gas is as high as about 1000xc2x0 C. In order to preheat the combustion air using a conventional metallic air preheater 200 with this high-temperature furnace exhaust gas, it is necessary first to lower the gas temperature to about 800xc2x0 C. by using dilution air to prevent the air preheater from being burned out.
When furnace exhaust gas of about 800xc2x0 C. is used, the temperature of the obtained preheated air is about 600xc2x0 C. at the most. Therefore, the heat recovery efficiency is poor, and a large amount of excess heat is wasted. It might be thought that this excess heat can be used to preheat limestone, which is a substance to be heated. However, since this system is used for a process in which direct contact of the substance to be heated with high-temperature combustion gas is unfavorable, the preheating of the substance should also be accomplished indirectly. Thus, it is difficult to improve the thermal efficiency in this way. Further, the amount of exhaust gas discharged from the system increases due to the dilution air, which results in a further increase in heat loss.
It is an object of the present invention to provide an indirect heating furnace which can improve the thermal efficiency dramatically and can increase the throughput significantly in a solids heating process in which direct contact of the solids with high-temperature combustion gas is unfavorable.
To achieve the above object, the present invention provides the following indirect heating furnaces:
[1] An indirect heating furnace for heating a substance in a reaction tube with a high-temperature combustion gas that does not contact that substance, characterized in that the heating takes place in a stationary ceramic reaction tube, and the combustion device supplying the heating high-temperature combustion gas into the furnace is comprised of at least one pair of regenerative burners.
[2] The indirect heating furnace as described in the above item [1], characterized in that the temperature of the high-temperature combustion gas supplied into the furnace is 1000xc2x0 C. or higher.
[3] The indirect heating furnace as described in the above item [1] or [2], characterized in that the furnace has a screw conveyor for transporting the substance to be heated through the reaction tube, and the essential portion of the screw conveyor is made of ceramics.